It has been customary in the electronic article surveillance industry to apply EAS markers to articles of merchandise. Detection equipment is positioned at store exits to detect attempts to remove active markers from the store premises, and to generate an alarm in such cases. When a customer presents an article for payment at a checkout counter, a checkout clerk either removes the marker from the article, or deactivates the marker by using a deactivation device provided to deactivate the marker.
Known deactivation devices include one or more coils that are energizable to generate a magnetic field of sufficient amplitude to render the marker inactive. One well known type of marker (disclosed in U.S. Pat. No. 4,510,489) is known as a "magnetomechanical" marker. Magnetomechanical markers include an active element and a bias element. When the bias element is magnetized in a certain manner, the resulting bias magnetic field applied to the active element causes the active element to be mechanically resonant at a predetermined frequency upon exposure to an interrogation signal which alternates at the predetermined frequency. The detection equipment used with this type of marker generates the interrogation signal and then detects the resonance of the marker induced by the interrogation signal. According to one known technique for deactivating magnetomechanical markers, the bias element is degaussed by exposing the bias element to an alternating magnetic field that has an initial magnitude that is greater than the coercivity of the bias element, and then decays to zero. After the bias element is degaussed, the marker's resonant frequency is substantially shifted from the predetermined interrogation signal frequency, and the marker's response to the interrogation signal is at too low an amplitude for detection by the detecting apparatus.
Prior application Ser. No. 08/801,489 (which is commonly assigned with the present application) discloses a marker deactivation device in which conductive coils are driven with a constant amplitude sinusoidal signal to generate an alternating magnetic field at and for some distance above a top surface of the deactivation device. A magnetomechanical marker swept over the top of the device is exposed to a decaying-amplitude alternating field as the marker exits the region above the deactivation device, resulting in degaussing of the marker bias element and deactivation of the marker.
In another type of deactivation device, such as is disclosed in U.S. Pat. No. 5,493,275 (which has a common inventor and common assignee with the present application), the deactivation device includes a circuit for detecting the presence of a marker to be deactivated. When the presence of the marker is detected, a coil drive circuit is triggered to generate a decaying-amplitude alternating signal which is applied to a deactivation coil. Because the driving signal applied to the coil in the latter type of device is itself a decaying signal, it is not necessary to sweep the marker past the deactivation device, and effective deactivation is accomplished even if the device is simply placed on top of the deactivation device.
It is also known to trigger the coil drive circuit in response to a button or switch actuated by the checkout clerk.
Although the types of deactivation devices described above may be satisfactorily employed for their intended purpose, it would be desirable to provide a deactivation device for magnetomechanical markers which does not require sweeping the marker past the deactivation device or detecting the presence of the deactivation device or triggering by a human operator.